"Design of video conference gateway based on H.323 and SIP protocol
With the continuous development and maturity of VoIP technology, video conference system has become an important aspect of computer communication technology. Aiming at the design and implementation of video conference system supporting multiple protocols, this paper analyzes the characteristics and differences of two mainstream protocols H.323 and SIP in the field of visual communication, and gives a gateway design scheme to solve the protocol conversion between H.323 and sip. key word H.323; SIP; MC; Video conference gateway; Protocol conversion one Introduction with the continuous development and popularization of broadband network, people's demand for video communication is increasing. Two major protocols that play a leading role in the field of video communication include: ITU-U (International) TelecommunicaTIons Union – TelecommunicaTIon StandardizaTIon H.323 protocol recommended by sector, Telecommunication Standards Department of the International Telecommunication Union and Engineering Task Force, SIP protocol (session) recommended by Internet Engineering Task Force Initiation Protocol, Session initiation protocol), due to the era background, existing development and characteristics of the two protocols, they will coexist for a long time in the field of video communication, forming a pattern of mutual competition and complementarity. Therefore, an important work in visual communication is to solve the protocol conversion between H.323 and sip, that is, the interconnection of visual terminals and devices using these two protocols. Video conference system is an important application in visual communication. When designing video conference system, gateway design is used to realize the integration between different networks. H. 323 protocol family adopts multipoint control unit (MCU) to control and manage the conference system. It is composed of multipoint controller (MC) and multipoint processor (MP), and uses three different methods: centralized, decentralized and hybrid to realize the multipoint control and management of the conference. In SIP protocol, multi-party conference is established through session Notification Protocol (SAP) and session description protocol (SDP). There are some differences in mechanism between the two, so the support of video conference gateway is needed to realize interworking. two H. Characteristic analysis of 323 protocol and SIP protocol 2.1 H. The main body of H.323 protocol has become more and more stable, and its basic framework has been widely adopted. It defines four basic functional units: user terminal, gateway, gatekeeper and multipoint control unit (MCU). The user terminal can carry out real-time and bidirectional voice and video communication with other H.323 entities. It can realize the following functions: (1) Signaling and control: it supports H.245 protocol and can realize channel establishment and capability negotiation; Support Q.931 protocol and realize call signaling channel; Support Ras protocol and realize communication with gatekeeper; （ 2） Real time communication: support RTP / RTCP protocol（ 3） Codec: support various mainstream audio and video codec functions. Gateway provides a way to connect circuit switched network (SCN) and packet switched network. It completes the call establishment and control functions on different networks. The gatekeeper provides call control services to the h.323 terminal to complete the following functions: address translation, permission access, conference control and management, bandwidth control and management, outgoing call management, domain management, etc. Multipoint control unit (MCU) completes the conference control and management functions. It is composed of multipoint controller (MC) and multipoint processor (MP). The multipoint controller provides the control function of multipoint conference. In multipoint conference, the multipoint controller establishes an H.245 control connection with each h.323 terminal to negotiate the media communication type; Multipoint processor provides media switching and mixing functions. H. 323 supports centralized and decentralized multipoint control and management. In the centralized working mode, the multipoint processor (MP) and each h.323 terminal in the conference establish a media channel to uniformly process the received audio stream and video stream, and then send it back to each terminal. In the decentralized working mode, each terminal should support the function of multipoint processing and realize the multipoint transmission of media stream. two point two SIP Protocol SIP is an application layer control (signaling) protocol for multimedia communication. It can establish, change and terminate multimedia sessions. There are two main components in SIP protocol: user agent (UA) and network server. User agent (UA) is divided into user agent client (UAC) initiating SIP request and user agent server (UAS) responding to such request. Network servers are divided into: redirection server, proxy server and registration server. The usual SIP operation involves a SIP user agent client (UAC) initiating a request, a SIP proxy server acting as the location discovery agent for the end user, and a user agent server (UAS) accepting the call. A successful SIP invitation includes two request information: invite information and ACK information. The redirection server sends back a sip-url (SIP) containing the callee's location Uniform Resource Locations, SIP unified resource location, such as SIP:
[email protected] ）To process invite information. The proxy server completes the application layer routing of SIP requests and responses. The registration server is used to record the user's sip-url address information and its corresponding IP address information. three As a video conference gateway, the design of video conference gateway needs to support both H.323 and sip protocols. On one side of H.323 protocol, it acts as a multipoint controller (MC); On one side of SIP protocol, it plays the role of conference server. In the video conference gateway, the translation and conversion from one signaling message to another signaling message are completed, and the functional mapping between the two is completed equally. However, in H.323 protocol and SIP protocol, some functions are only provided in one protocol, but can not be supported in another protocol; One process in one protocol may be mapped to several processes in another protocol, which are the problems that video gateway needs to solve. Several key technical problems in the design of video conference gateway are introduced below. Figure 1 Networking diagram of video conference gateway 3.1 Signaling message mapping signaling message mapping and transformation is one of the key technologies of video conference gateway. Message mapping generally includes two aspects: one is the transformation of message coding, such as the transformation of binary coding mode and text mode; The second is the mapping of semantic rules in the message. For example, after the terminal of the SIP party sends an invite request message to the gateway, the gateway generates a call setup or access request (ARQ) message at the H.323 party. H. 323 protocol family includes call establishment, call control, media channel control and many other decentralized protocols and processes to complete multimedia communication. However, there is no strict classification in SIP protocol. Therefore, in the process of message mapping and conversion, there may be a SIP signaling message corresponding to several signaling message processes on the H.323 side. Table 2 shows the mapping and conversion of command messages between H.323 protocol and SIP protocol in the process of establishing, inviting and joining video conference. three point two Media capability negotiation uses H.245 protocol in H.323 protocol to complete the negotiation process of media capability. In the conference, the multipoint controller (MC) collects the media capability information submitted by each terminal, and selects the most common media type as the media capability set for each member to communicate in the conference. In SIP, session description protocol (SDP, which is a part of SIP message body) is used to realize the negotiation process of media capability. Therefore, if the H.323 client terminal initiates the conference, the video conference gateway must determine the media type of the conference according to the media capability information of the client terminal contained in the H.245 message, and then encapsulate these media capability information into the SDP message and send it to the SIP client terminal. If the SIP client terminal sends the invitation first, the conference gateway uses the information about the client terminal capability contained in the SDP message to determine the media capability type of the conference. three point three The management and control of video conference integrates the functions of protocol converter, video conference controller and manager in the video conference gateway. Before H.323 client terminal initiates a conference request, it must register with the gateway guard. When initiating a request, the H.323 client terminal first sends the message to the gatekeeper, and then the gatekeeper forwards the message to the conference gateway. Finally, the conference request sent by the client terminal is processed uniformly through the conference control and manager of the video conference gateway, and decides to accept or reject the request of a client terminal, as well as the media type of the whole conference. On the SIP side, the SIP client terminal must first register with the server. When the client terminal sends a request, it first sends the message to the server, and then the server forwards it to the conference gateway. In the conference gateway, the received message is converted into the corresponding signaling mechanism in accordance with H.323 rules through the signaling message mapping principle, and then sent to the conference control and manager for centralized processing. three point four Introduction to video conference process in order to better illustrate the role and workflow of video conference gateway in cross protocol conference system, we take a conference request initiated by H.323 client terminal as an example to describe the processing process of video conference gateway for two protocol signaling messages (as shown in Figure 2). Figure 2 H. 323 process of inviting SIP user agent by client 4 Implementation of video conference gateway the interior of video conference gateway is mainly composed of SIP protocol processing module, protocol conversion module, video conference control and management module and H.323 protocol processing module. SIP protocol processing module is a video conference gateway, which is used to listen for signaling messages sent by SIP entities (SIP user agent, SIP server, etc.). After necessary processing, these signaling messages are sent to the protocol conversion module for protocol conversion. Protocol conversion module is a key part in the implementation of video conference gateway. In the implementation process, it needs to translate and convert one by one according to the signaling message mapping table between the two protocols. Of course, for those signaling messages that exist in one protocol but are not supported in another protocol, appropriate ignore and shielding measures should be taken, and error report messages should be returned. The converted signaling messages are sent to the processing module of their respective protocols for further processing, and can also be sent to the video conference control and management module to complete the organization, management and control of the conference and its members. Video conference control and management module is the core of video conference gateway. The video conference control and management module not only processes the signaling messages sent by conference members, but also processes the video media stream. In the implementation of this part, the different characteristics of the two protocols in video conference function should be fully considered and treated differently, so as to reduce the workload in signaling translation and identification. At the same time, RTP / RTCP protocol is adopted in the processing of video stream. Firstly, the media streams of each client terminal are gathered together, and then transmitted to each client terminal after recombination. H.323 The protocol processing module is used to listen for signaling messages from H.323 protocol parties. Then send the signaling message to the video conference control and management module or protocol conversion module for the next step. The implementation needs the bottom support of H.323 protocol stack. We're in red Hat It is implemented in C / C + + language under Linux 9.0 operating system. We also use some implementation technologies of video conference management and control in openmcu (open source H.323 conference server). five Summary This paper briefly introduces the design and implementation of video conference gateway supporting multi protocol. We find that the protocol conversion of video conference gateway can only realize the translation of some simple signaling messages, and there is no good fault tolerance and neglect mechanism. Of course, in order to design and implement a video conference gateway with more perfect and efficient functions, further research and development need to be done in strengthening the functions of conference management and conference control., Technology Zone
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